In conventional rice processing the moisture content of the rice undergoing treatment is extremely important and must be carefully controlled to effectively gelatinize the rice starch present and to enhance overall processability and integrity of the rice product. Typically, the heat required to properly gelatinize and cook the rice is supplied to the rice through contact with excess water or steam. Exemplary of such a process is the conventional parboiling of rice.
The parboiling of rice is known to be a hydrothermic process by which desirable properties are imparted to the raw rice grain. One very important benefit of rice parboiling is in the resultant texture of the rice kernel which can be modified in such a way as to enable the consumer to obtain a finished, cooked rice product exhibiting a highly desirable texture. In some cases it is desirable to prepare rice which when cooked is firm but not sticky and is composed of separate grains. At other times, a different final texture may be desired. In all cases, however, the finished or cooked rice should substantially consist of intact kernels with little or no disintegrated or broken kernels.
Conventional rice parboiling processes typically include three basic steps; namely (1) soaking in water to obtain a stable moisture content (2) steaming, and (3) drying. The steaming or heat treatment step is conducted in the presence of water, steam or some other aqueous medium during which the rice is gelatinized. The moisture content of the rice starch prior to heat treatment together with the process conditions of temperature, pressure and heat transfer media are important factors in determining the level and effectiveness of gelatinization.
It should be appreciated that gelatinization of rice kernels is typically referred to as the irreversible swelling of the starch granules due to the effect of water and heat, resulting in loss of birefringence when observed under polarized light. Such gelatinization can be considered a melting process consisting of three basic steps; namely, (1) the diffusion of water into the starch granule (2) helix-coil transition of the starch molecule requiring varying levels of moisture and energy, and (3) swelling of the granules. It has been observed that gelatinization of starch is directly proportional not only to the moisture content of the rice grain prior to heat treatment, but also to the process temperature and process time. As the moisture content increases the amount of energy required to achieve a prescribed level of gelatinization decreases in terms of time and temperature . The higher the processing temperature the higher the level of gelatinization achieved for a given moisture content and process time. Correspondingly, the longer the process time above the minimum required moisture content and process temperature, the greater the degree of gelatinization.
Typically, at a moisture content below approximately 20%, the temperature necessary to effect gelatinization exceeds the temperature which causes carmelization or burning of the starch thereby negating any desired gelatinization effect. That is, when the process temperature exceeds approximately 140.degree. C. The acceptable upper limit of moisture content can be determined by the amount of water that rice starch will absorb at a temperature below which no gelatinization occurs. For most typical rice grains this "soaking temperature" is below about 70.degree. C. and rice starch will absorb moisture up to a maximum of about 50% water.
As one skilled in the art will appreciate, gelatinization of starch can be controlled to varying levels just as any chemical reaction can be controlled to any degree of completion. Gelatinization of starch, in the grain or as a free starch granule, is a first order chemical reaction. Accordingly, the gelatinization reaction is dependent upon the temperature at which the reaction is conducted, the time for which it is conducted and the concentration of water available for reaction. Thus, the degree of gelatinization can be controlled by varying the process time and temperature and the moisture content of the rice. As pointed out above, gelatinization has a very considerable impact on the subsequent processability and textural quality of the rice kernel. Thus, the ability to effectively control gelatinization and therefor the overall parboiling process is highly desirable.
It has also bcen found that heretofore known parboiling processes exhibit certain limitations or deficiencies. More particularly, paddy rice (that is, with the hull remaining on the rice grain) is typically used as the raw material for the parboiling process. The rice hull acts not only to prevent or minimize the escape of nutrients from within the rice kernels but also protects the rice grains against the detrimental effects of elevated temperatures and the steam processing environment required to gelatinize the starch granules.
While the known parboiling techniques continue to enjoy widespread acceptance and success in the processing of paddy rice, they are not suitable for use in connection with white rice or brown rice from which the hulls have been removed. In the conventional parboiling process, the use of steam has been found to cause the surface of both white and brown rice to become saturated with water thereby destroying the grain integrity and intactness and resulting in overprocessing of the grain surface. Such steaming (or even just prolonged soaking) causes process or heat damage and decreases product quality and yield to undesirable levels.
It can thus be readily appreciated that a real need exists for a process to treat a wide variety of rice grains including white and brown rice without experiencing the deleterious effects of conventional steam or aqueous processing. Such a process would confer on the art the significant advantage of increasing not only the effectiveness of known processes for the treatment of rice but would also permit the processing of certain classes, types and varieties of rice which heretofore have not been susceptible to parboiling or other treatment at elevated temperatures.